Method of and apparatus for attaining focusing following variation in magnification in electron microscope

ABSTRACT

A method of and apparatus for attaining the focusing following a variation in the magnification in an electron microscope having an image producing lens system consisting of three or more lenses or generally n lenses including at least an objective lens, an intermediate lens and a projective lens. Means are provided in the apparatus so that, when the value of excitation current for the magnification varying lens is varied to vary the magnification, the excitation current supplied to the focus adjusting lens can be set at a value corresponding to the variation in the magnification, whereby the focusing following the variation in the magnification by the magnification varying lens as well as the adjustment of the intensity of illumination can be achieved by a single regulating operation.

United States Patent 91 Akahori et al. Feb. 6, 1973 54 METHOD OF ANDAPPARATUS FOR 3,535,514 10/1970 Cardile ..250/49.5 A ATTAINING FOCUSINGFOLLOWING 3 32%: gflgg 3 VARIATION IN MAGNIFICATION IN 315091275 4/1970Deeley......................::::::250/49:5 A

ELECTRON MICROSCOPE Inventors: Hiroshi Akahori; Morioki Kubozoe;Toshihiro Furuya; Sadayasu Ueno, all of Katsuta, Japan Assignee:Hitachi, Ltd., Tokyo, Japan Filed: Aug. 21, 1970 Appl. No.: 65,996

Foreign Application Priority Data.

- Feb. 13,1970 Japan ..-....45/40945 May 15, 1970 Japan ..4 5/l2377 US.Cl. 250/49.5 A, 250/495 D Int. Cl ..H0lj 37/10 Field of Search..250/49.5 A, 49.5 C, 49.5 D

v References Cited UNITED STATES PATENTS POWER SUPPLY 4/1970 Deeley..250/49.5 A,

PrimaryExaminep-Archie R. Borchelt Assistant Examiner-C. E. ChurchAttorney-Craig, Antonelli, Stewart & Hill 57 ABSTRACT 1 tion, wherebythe focusing following the variation in the magnification by themagnification varying lens as well as the adjustment of the intensity ofillumination can be achieved by a single regulating operation.

8 Claims, 7 Drawing Figures PATENTEDFEB 61975 3.715.582

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ATTORNEYS METIIOI) OF AND APPARATUS FOR ATTAINING FOCUSING FOLLOWINGVARIATION IN MAGNIFICATION IN ELECTRON MICROSCOPE BACKGROUND OF THEINVENTION tion is varied by varying the value of excitation currentsupplied to the magnification varying lens in an electron microscope.

2. Description of the Prior Art In an electron microscope, themagnification must be suitably varied for the observation of variouskinds of specimens or for the observation of. those specimens undervarious states. The manipulation for the variation in the magnificationin an electron. microscope includes varying the value of excitationcurrent supplied to the magnification varying lens for varying themagnification and then varying the value of excitation current suppliedto the focus adjusting lens for attaining the focusing thereby toproduce an enlarged image of a specimen on a final image screen. In theelectron microscope, the intensity of illumination of the final imagemust be kept constant since the intensity of illumination of the imagevaries in inverse proportion to the square of the magnification with anincrease or decrease in the magnification.

By way of example, in an electron microscope having a three-stage imageproducing lens system consisting of an objective lens, an intermediatelens and a projective lens, the intermediate or projective lens acts asthe magnification varying lens, while the objective lens acts as thefocus adjusting lens. l-Ieretofore, a microscopist hasmanually adjustedthe value of excitation current supplied to the focus adjusting lens andcondenser lens while observing the final image produced on thefluorescent screen.

, However, such a focusing system has been defective in that anelongated period of time is required for the focus adjustingmanipulation and, since the specimen is exposed to the electron beam forsuch a long period of time, the surface of the specimen is damaged tosuch an extent that it is no longer possible to observe the specimenunder ordinary conditions. This defect has been especially marked in thecase of a specimen such as a high molecular compound which is easilyaffected by heat. Further, it has been very difficult with the prior artfocusing system to attain the focusing following a variation in themagnification in the case of observation of a specimen such as a pieceof tissue with a so-called low magnification of the order of severalthousand times or less, and the manipulation for the proper gfocusinghas required an elongated period of time. Thus, this defect hasaggravated the above-described defect.

SUMMARY OF THE INVENTION 7 method.

Another object of the present invention is to provide an apparatus forattaining the focusing following a variation in the magnification in anelectron microscope in which means are provided so that, when the valueof excitation current supplied to the magnification varying lens isvaried to vary the magnification, the excitation current supplied to thefocus adjusting lens can be set at a value corresponding to the abovevariation in the magnification to attain the focusing in simultaneousrelation with the manipulation for the variation in the magnification,and at the same time, the excitation current supplied to the condenserlens can also be set at the value corresponding to the above variationin the magnification to adjust the intensity of illumination, wherebythe variation in the magnification can be always carried out in thestate in which the focusing is properly attained and the intensity ofillumination is completely adjusted.

A further object of the present invention is to provide an electronmicroscope having an image producing lens system consisting of three ormore lenses or generally ri lenses including at least an objective lens,an intermediate lens and a projective lens, in which means are providedto set the excitation current supplied to the focus adjusting lens at avalue corresponding to a variation in the value of excitation currentsupplied to the magnification varying lens so that the focusingfollowing the variation in the magnification can be carried out by asingle regulating operation thereby to eliminate the troublesomemanipulation involved in the handling of the electron microscope.

A yet further object of the present invention is to provide, in-anelectron microscope having an image producing lens system consisting ofthree or more lenses or generally n lenses including at least anobjective lens, an intermediate lens'and a projective lens, a method ofvarying the magnification of the electron lens comprising fixing thefocal length of the intermediate lens and varying the value ofexcitation current supplied to the lens or lenses disposed in the stageor stages lower than the intermediate lensfor carrying out the variationin the magnification.

A still further object of the present invention is to provide, in anelectron microscope having an imageproducing lens system consisting ofthree or more lenses or generally n lenses including at least anobjective lens, an intermediate lens and a projective lens, a method andapparatus for varying the magnification of the electron microscope inwhich the value of excitation current supplied to the intermediate lensis varied stepwise while the value of excitation current supplied to thelens or lenses disposed in the stage or stages lower than theintermediate lens is continuously varied at a portion corresponding tothe stepped portion of the intermediate lens excitation current so as tocarry out the variation in the magnification.

A common object of the present invention is to provide a novel method ofvarying the magnification of an electron microscope and an apparatuscapable of attaining the focusing following the variation in themagnification by a single regulating operation in simultaneous relationwith the above variation in the magnification.

Other objects, features and advantages of the present invention will bereadily apparent from the following detailed description taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic view showingthe relative position of electron lenses and the positional relationbetween image planes in an electron microscope having a four-stage imageproducing lens system consisting of an objective lens, an intermediatelens, a first projective lens and a second projective lens. FIG. 2 is ablock diagram of an embodiment of the present invention havinginterlocking control means for the simultaneous control of the values ofexcitation current supplied to electron lenses in an electron microscopehaving an image producing lens system consisting of four or more lensesor generally n lenses including at least an objective lens, anintermediate lens, a first projective lens and a second projective lens.

FIG. 3 is a graph showing the relation between objecand the principalplane 2 of the objective lens and the distance between the principalplane 2 and the image forming plane 3 of the objective lens Orespectively. Suppose further that f,, a,, b,; f,.,, a,.,, b and 5 fpz,a bpz are those of the intermediate lens I,,,, first tive lensexcitation current and-intermediate lens.ex 1

citation current when, in an electron microscope having an imageproducing lens system consisting of three or more lenses or generally nlenses including at least an objective lens, an intermediate lens and aprojective lens, the value of excitation current supplied to theelectron lens or lenses disposed in the stage or stages lower than andincluding the projective lens is kept constant.

FIG. 4 is a graph showing themann'er of variation in the values ofexcitation current supplied to electron lenses for illustrating anembodiment of the method of varying the magnification according to thepresent invention when it is applied to an electron microscope having afour-stage image producing lens system consisting of an objective lens,an intermediate lens, a first projective lens and a second projectivelens.

FIG. 5 shows a modification of the embodiment shown inFlG. 2. I

FIGS. 6 and 7 are modifications of parts of the embodiment shown in FIG.5.

' DESCRIPTION OF THE PREFERRED EMBODIMENTS The relative position ofelectron lensesand the positional relation between image planes in anelectron microscope having a four-stage image producing lens systemconsisting of an objective lens, an intermediate lens, a firstprojective lens and a second projective lens will be described withreference to FIG. 1. In FIG. 1, a specimen to be observed is designatedby the reference numeral 1. The objective lens 0,, has aprincipal plane2 and an image forming plane 3. Theintermediate lens 1,, has a principalplane 4 and an image forming plane 5. The image forming plane 5 of theintermediate lens Int corresponds at the same time to the object planefor the first projective lens P, disposed in the next stage. The firstprojective lens P hasa principal plane 6 and an image'forming plane7.The image forming plane 7 of the first projective lens P corresponds atthe same time to the object plane for the second projective lens P,disposed in the next stage. The second projective lens P, has aprincipal plane 8and an image forming plane 9. Generally, a fluorescentscreen for the image observation'or the emulsion surface of aphotographic plate or film is disposed at the image forming plane 9 ofthe second projective lens P Suppose that f ',a,, and b, are the focallength of the objective lens O,,,,'the distance between the specimen 1projective lens P, and second projective lens P respectively, and u, vand w are the distances between the principal planes of the four lenses.Then, the following equations hold:

' l/a,,+ 1Ib,,=l/f 1 /fI l/a 'l' I/bp =1/fp l pz P2 /fP2 a, b u Pl 1 vP2 l! w Suppose further that M M,, M and Mp2 are the magnification ofthe objective lens O,,,, that of the intermediate lens I,,,, that of thefirst projective lens P and that of the second projective lens Prespectively. Then, the total magnification M is given by MT: a/ o X l/l X Pll Pl X P2 P2 o l MP1 P2 In the electron microscope, the variationin the magnification is generally carried out by varying the value ofexcitation current supplied to the intermediate lens I, thereby varyingthe focal length f, of the intermediate lens I,,,, that is, by shiftingthe position of the object plane 3 for the intermediate lens l,,,. Moreprecisely, when the distance a, is varied to a, 01,, the magnificationof the intermediate lens I is now given by M, b,/(a,'+ a and themagnification of the objective lens O is given by M (b a,)/a,,. Thus,the magnification at the portion including the objective lens O and theintermediate lens I, is varied from MD'MI t0 M 1 (b a,)/a,, b1/(Ll1 aHowever,

due to the fact that the image forming plane 3 of the 1 objective lens Ois shifted by --a,, the image would be out of focus unless the focallength f, of the objective lens O is varied correspondingly providedthat the distance a, is fixed.

Thus, with the system described above, the variation in themagnification by the intermediate lens results in blurring of the image,and therefore, the value of excitation current supplied to the objectivelens must be adjusted for attaining the proper focusing each time the.sure' to the electron beam for a long period of time.

With a view to overcome the above drawbacks, the present inventioncontemplates the provision of an electron microscope which is providedwith means for setting the excitation current supplied to the focusadjusting lens at a value corresponding to a variation in themagnification in simultaneous relation with the variation in themagnification by the magnification varying lens. The present inventioncontemplates also the provision of a novel method of carrying out thevariation in the magnification.

The method of varying the magnification according to the presentinvention will be described in detail hereunder.

l. At first, the variation in the focal length f, of the objective lens0,, when the magnification is varied by varying the value of excitationcurrent supplied to the intermediate lens I, in the relative positionshown in FIG. 1 will be discussed. In this case, the focal length f, ofthe intermediate lens 1,, is solely subject to variation. Thus, thefocal lengths f,., and f are fixed and the distances a a,, a bp a and bare kept constant. The total magnification M in this case is given by i=o MI MP1 MP2 K1 e where 1 l/ Pll Pl) ps P2)- Substituting a, and b, inthe equation (8) by those obtained from the equations (2) and M isexpressed MT: K1 ["UF'fl) lfr/ llfl The focal length f, is sought fromthe equation (9) as 0 u heft? The equation (12) is differentiated with Mto find the variation Af, of f], when f, is varied to vary M The resultis given by fa)fI 1/ 'flf' r' r (13) The above equation represents thevariation Af of the focal length f of the objective lens 0,, when thetotal magnification is varied by AM by varying the value of excitationcurrent supplied to the intermediate lens 1,

2. The next discussion is directed to the case in which themagnification is varied by varying the value of excitation currentsupplied to another lens, for example, the first projective lens P,instead of the intermediate lens l,,,. In this case, the focal length fpis solely subject to variation. Thus, the focal lengths f, and f arefixed and the distances a bp a and b are kept constant. The totalmagnification M in this case is given by 1 o' l' m' rz u/ 0 1)'( Pi/m)'( Pz/ pz) i( o l)'( l/ m) 1( Pr/ m) where [61 (hm/ 0 bra/ P2 Theequation (17) is differentiated with M to find the variation Af of f,when M is varied by varying fp The result is given by flv){ 1/ 2)' fo FT The above equation represents the variation Af of the focal length f,of the objective lens O when the magnification is varied by varying thevalue of excitation current supplied to the first projective lens P 3.The ratio between the equations (13) and (18) is sought to seek theratio between the variations Af of the focal length f, of the objectivelens 0,, when the magnification is varied to the same value.

For the sake of simplicity, it is assumed that u z v most cases.

when u V,fpz is constant and (h (b h Since the focal length f, of theintermediate lens I is used in the range in which it is smaller than uand the relation f, u holds at a high magnification, the followingequation can be obtained:

ft!) fp1 The above equation shows the fact that, when the mag nificationis varied by varying the focal length f, of the intermediate lens l,,,,the variation Af of the focal length f of the objective lens O isgreater by the magnification of the first projective lens l than whenthe magnification is varied by varying the focal length f of the firstprojective lens P In other words, the above equation implies that thevariation f, in the focal length f, of the objective lens for thevariation in the magnification is smaller when the magnification isvaried by varying the focal length f of the first projection lens thanwhen the magnification is varied by varying the focal length f, of theintermediate lens. The ratio between the two cases is about l/MOrdinarily the value of M is 10 to 20. Consequently, when themagnification is varied by varying the focal length f of the firstprojection lens, the

image falls within the depth of focus of the objective lens 0,, tobecome a focused or distinct image even without varying the focal lengthf, of the objective lens.

This applies also to an electron microscope having a three-stage imageproducing lens system consisting of an objective lens, an intermediatelens and a projective lens. In this case too, the variation in themagnification can be carried out in a substantially sharply focusedstate by varying the magnification by the projective lens while fixingthe value of excitation current supplied to the intermediate lens.

However, the variation in the magnification solely by the firstprojective lens is not practical because its magnification varying rangeis narrow.

In such a case, as will be described later with reference to FIG. 4, theentire magnification range of the electron microscope is divided into Nsections, at the boundaries of which the excitation current of theintermediate lens is varied discontinuously, but within each of which itis kept constant. Within each section the excitation current of the lensof a subsequent stage to the intermediate lens, that is, of the first orsecond projection lens is finely varied so that the magnification isvaried without a substantial variation in the focal length f, of theobjective lens, that is, in the substantially focused state.

In order to keep the focal length of the objective lens substantiallyconstant irrespective of the variation in the magnificatiomthe variationin the excitation current of the objective lens corresponding to thevariation in the excitation current of the intermediate lens, which hasbeen calculated or measured in advance, is imparted to the excitationcurrent of the objective lens in coordination with the switching of theexcitation current of the intermediate lens. In this manner themagnification can be varied over its entire range in a substantiallyfocused state.

The above description has referred to the method of varying themagnification of an electron microscope having a three-stage imageproducing lens system consisting of an objective lens, an intermediatelens and a projective lens.

FIG. 2 is-a block diagram of means for controlling the excitationcurrent supplied to electron lenses in an electron microscope having animage producing lens system consisting of four lenses or generally nlenses including at least an objective lens, an intermediate lens, afirst projective lens and a second projective lens As for the means forcontrolling the excitation current suppliedto electron lenses in anelectron microscope having a three-stage image producing lens system,the case in which the excitation current supplied to the lenses disposedin the stages lower than the third stage is fixed at a predeterminedvalue in FIG. 2 may be con sidered.

Referring to FIG. 2, the reference numerals 20, 21, 22 and 23 designatean objective lens excitation current control system, an intermediatelens excitation current control system, a first projective lensexcitation current control system, and a second projective lensexcitation current control system, respectively. The reference numeral24 designates generally an nth projective lens excitation currentcontrol system. The electron lens control systems 20, 2I 22, 23 and 24are supplied with current from a common stabilized power supply 25.

The objective lens excitation current control system 20 includes anobjective lens coil 0,,, a detecting resistor R, for detecting theexcitation current flowing through the objective lens coil 0,,, avoltage-dividing resistor R, for the detecting resistor R,, an erroramplifier D, such as a balanced DC amplifier for detecting andamplifying the difference between a voltage detected by thevoltage-dividing resistor R, and a reference voltage V,,,, and an outputcontrol means C, which is controlled by the output from the amplifier D,for controlling the excitation current flowing through the coil 0,,. Thelens excitation current flowing through the objective lens coil 0,, isconverted into a voltage by the combination of the current detectingresistor R, and the voltage-dividing resistor R,, and the differencebetween this voltage and the reference voltage V,,, is detected andamplified by the error amplifier D,. The output from the error amplifierD, is supplied to the output control means C, for stabilizing thecurrent flowing through the excitation coil 0,,. That is, the lensexcitation current is maintained at a predetermined value depending onthe value of the detecting resistor R,. The setting of the lensexcitation .current flowing through the objective lens coil 0,, can bevaried by varying the resistance of the detecting resistor R, or thevoltage division ratio of the voltagedividing resistor R,'. In thepresent embodiment the value of the excitation current flowing throughthe objective lens coil 0,, is varied by varying the voltage divisionratio of the voltage-dividing resistor R, connected in parallel with thedetecting resistor R,. For this purpose, the voltage-dividing resistorR, may have a variable resistance so that it may be varied continuouslyor the voltage-dividing resistor R, may be divided stepwise as shown inFIG. 2 so that its resistance may be changed over by a changeover switch30,.

The operation of the remaining electron lens excitation current controlsystems 21, 22, 23 and 24 is similar to the operation of the objectivelens excitation current control system 20, and suffixes I, P,, P, and P,are attached to the corresponding components of the intermediate lensexcitation current control system 21, first projective lens excitation,current control system 22, second projective lens excitation currentcontrol system 23, and nth projective lens excitation current controlsystem 24, respectively.

The variation in the values of excitation current in the electron lensexcitation current control systems is carried out by an interlockingchange-over means 40 such as a rotary switch which changes over thevoltage division ratios R of the detecting resistors which are preset tosupply required current values in a fixed relationship with each other.As an example, when the excitation currents for the electron lensesdisposed in the stages lower than and including the first projectivelens P, are kept at respective fixed values and the focal length f, ofthe intermediate lens I, is varied to vary the magnification of theelectron micriscope shown in FIG. 2, a corresponding variation Af, ofthe focal length f, of the objective lens 0,, will be as shown in theequation (13). It is known that the focal length of a magnetic lensgenerally varies in inverse proportion to the square of the excitationcurrent. Suppose, in this case, that I, and I, are the excitationcurrent for the objective lens 0,, and the excitation current for theintermediate lens I respectively. Then, the following equation isderived from the equation (12):

o 1+[ 2 l 3)] (21) where C,, C and C are constants. This relation isillustrated in FIG. 3.

Therefore, the voltage division ratio of the voltagedividing resistor R,for the detecting resistor R, in the intermediate lens excitationcurrent control system 21 and the corresponding voltage division ratioof the voltage-dividing resistor R, for the detecting resistor R, in theobjective lens excitation current control system 20 may be determined onthe basis of the relation between the intermediate lens excitationcurrent I, and the objective lens excitation current I, shown in FIG. 3,and the change-over switches 30, and 30. for the changeover of thesevoltage division ratios may be mechanically coupled to each other by theinterlocking changeover means 40 as to attain the focusing following thevariation in the magnification by a single regulating operation. Theinterlocking change-over means 40 may be disposed between the objectivelens excitation current control system 20 and the intermediate lensexcitation current control system 21 or it may be arranged tosimultaneously control the values of all the electron lens excitationcurrents as shown in FIG. 2. In this latter case, the change-over of thevalues of excitation current for the lenses disposed in the stages lowerthan the intermediate lens I may be such that the excitation currentsare kept at the same values in spite of changeover of the steps forvarying the voltage division ratio of the voltage-dividing resistors R.For example, the change-over steps may be connected to the samevoltage-dividing point so as to show the same resistance.

A condenser lens excitation current control system 26 is so arrangedthat the excitation current for a condenser lens coil C is set at avalue corresponding to the variation in the value of excitation currentfor the intermediate lens coil O,,, in the intermediate lens excitationcurrent control system 21 in order to adjust the intensity ofillumination of the image following the variation in the magnification.Since the intensity of illumination of the final image can be keptalways constant in spite of the variation in the magnification,photographing, especially continuous photographing can be done withoutany fear of underexposure or overexposure.

The above description has referred to the simultaneous control means forthe electron lens excitation currents on the basis of the discussiongiven in paragraph I.

The following description is directed to a method and apparatus forvarying the magnification on the basis of the discussion given inparagraphs 2 and 3.

FIG. 4 is a graphic illustration of one form of the method of varyingthe magnification in an electron microscope having a four-stage imageproducing lens system consisting of an objective lens an intermediatelens I,,,, a first projective lens P, and a second projective lens P InFIG. 4, the horizontal and vertical axes represent the magnification andthe lens excitation current, respectively. As will be apparent from FIG.4,

the value of excitation current for the intermediate lens 1,, is variedstepwise and the value of excitation current for the first projectivelens P or second projective lens P, is varied continuously or stepwiseat portions corresponding to the stepped portions of the intermediatelens excitation current'for varying the magnification. Points bearingthe marks on the characteristic curves for the first and secondprojective lenses P and I represent the current values varied stepwise.

In the magnification range A of from 600 to 3,000 times, the values ofexcitation current for the intermediate lens I, and first projectivelens P are fixed at about 2.17A and 0.83A, respectively, while the valueof excitation current for the second projective lens P is variedcontinuously or discontinuously (stepwise) from about 2.67A to 4.17A. PP P represent the stepwise varying values of excitation current for thesecond projective lens P and P P P represent the settings of excitationcurrent for the objective lens 0 corresponding to the stepwise varyingvalues of excitation current for the second projective lens P Therefore,the voltage division ratios of the voltagedividing resistors R for thedetecting resistors R in the respective electron lens excitation currentcontrol systems may be set to have a relation as described above andthese voltage-dividing resistors may be mechanically coupled to eachother by the interlocking change-over means 40 shown in FIG. 2. Theabove arrangement applies also to the magnification ranges describedbelow.

In the magnification range B of from 3,000 to 30,000 times, the valuesof excitation current for the intermediate lens I and second projectivelens P are kept constant, while the value of excitation current for thefirst projective lens P is varied continuously or stepwise. In thisrange, the value of excitation current for the objective lens 0,, isvaried in a manner as shown.

In the magnification range C of from 30,000 to 300,000 times, the valuesof excitation current for the intermediate lens I, and second projectivelens P are kept constant, while the value of excitation current for thefirst projective lens P is varied continuously or stepwise. In thisrange, the value of excitation current for the objective lens 0,, isvaried in a manner as shown.

It will be apparent from FIG. 4 that the value of excitation current forthe objective lens O is varied stepwise in a relation similar to thestepped variation of the value of excitation current for theintermediate lens 1, but the variation in the value of excitationcurrent for the objective lens 0,, at the portions corresponding to thestepped portions of the excitation current for the intermediate lens I,is very little compared with the variation in the value of excitationcurrent for other electron lenses.

It will be understood that the variation in the magnification accordingto the socalled Zoom lens system can be carried out by an arrangement inwhich the values of excitation current for the respective electronlenses are varied in a relation as shown in FIG. 4 and an interlockingchange-over means 40 as shown in FIG. 2 is used for the simultaneouscontrol of the variation of these current values.

While the above description has referred to the case of varying thevalues of excitation current by varying the resistances of the detectingresistors or the voltage division ratios of the voltage-dividingresistors in the respective excitation current control systems, such maybe carried out by varying the voltage value of the reference voltagesource V Another embodiment of the present invention in which the valueof reference voltage is varied to vary the value of excitation currentwill be described with reference to FIG. 5. Although a three-stage imageproducing lens system consisting of an objective lens 0, an intermediatelens I, and a projective lens P is shown in FIG. by way of example, itwill be apparent for those skilled inthe art that the present embodimentis also applicable to an n-stage lens system. The suffixes O, I and Pare attached to components belonging to excitation current controlsystems 200, 210 and 220 for the objective lens 0 intermediate lens I,,,and projective lens P, respectively. The reference numerals 50 and 60designate a source of reference voltage and a voltage-dividing resistorgroup, respectively, which are common to the electron lens excitationcurrent control systems 200, 210 and 220. A change-over means S for theobjective lens excitation current control system 200 includes achange-over switch 70,, and a plurality of stationary contacts 0,, 0,,0,. A change-over means S, for the intermediate lens excitation currentcontrol system 210 includes a change-over switch 70, and a plurality ofstationary contacts 1,, I 1,. Similarly, a change-over means S for theprojective lens excitation current control system 220 includes achange-over switch 70 and a plurality of stationary contacts P,, P P,.The voltage-dividing resistor group 60 has a plurality ofvoltage-dividing terminals T T T,, T,,.

The precision of the setting of the value of excitation current isdifferent in each electron lens, but a very high degree of precision isrequired for every electron lens. For example, in order to eliminate anyfluctuation inthe intensity of illumination of the image as well as in Ythe focal point when the magnification is varied in a lens systemconsisting of a plurality of stages, the values of excitation currentmust not deviate beyond 0.001 percent in the case of the objective lensand 1 percent in the case of the condenser and projective lenses.Therefore, the voltage-dividing resistor group 60 used for the settingof the current value requires such a number of resistors which willcorrespond to the very finely divided units of the current value and thein dividual resistors must also be of high precision. Taking the abovepoint into consideration, the voltage-dividing resistor group 60 iscomposed of a series connection of n unit resistors having the samerated resistance R, and the stationary contacts 0,, O 0,; 1,, I 1,; andP,, P P, of the respective change-over means 8,, S, and S, are connectedto the voltage-dividing terminals corresponding to the desired currentsettings in a manner as, for example, shown in FIG. 5. When, forexample, the change-over switch 70, of the change-over means Sp for theprojective lens excitation current control system 220 is brought intocontact with the stationary contact P,, the input terminals of an erroramplifier D, is connected through the change-over switch 70, andstationary contact P, to the voltage-dividing terminal T, of thevoltage-dividing resistor group 60 connected to the reference voltagesource 50. Thus, a reference voltage which is S/n of the voltage V,, ofthe reference voltage source 50 is applied to the error amplifier DSimilarly, a voltage which is any desired division of the referencevoltage V,, can be obtained when each of the change-over switches 70,,and 70, is brought into contact with a suitable stationary contact.Therefore, the variation in the magnification can be always carried outin a sharply focused state when these stationary contacts are previouslyconnected to give respective fixed voltages as described in the firstembodiment and the change-over switches 70,, 70, and

70,. are simultaneously changed over by an interlocking change-overmeans 40. Two or three of the stationary contacts in the change-overmeans 5,, S, and S, are connected to the same voltage-dividing terminalof the voltage-dividing resistor group 60 as shown in FIG. 5. In orderto deal with possible fluctuation in the reference voltage value appliedto the error amplifier D,,, D, or D, when these stationary contacts areconnected to the same terminal, the input resistors for the erroramplifiers must have a sufficiently large resistance.

When it is required that the divided voltage of the reference voltageV,, applied to the error amplifier has a more precise value lyingintermediate between the divided voltage values appearing at theadjacent terminals T, and T of the voltage-dividing resistor group 60, aseries circuit including ten resistors each having a resistance of 10Rmay be connected in parallel with the resistor R lying between thevoltage-dividing terminals T, and T as shown in FIG. 6 or a seriescircuit including ten resistors each having a resistance of R/l0 may beconnected in series across the terminals T, and T,,, as shown in FIG. 7.By this arrangement, the excitation current can be set at a value whichis one place higher in precision than that obtained with the arrangementshown in FIG. 5.

We claim:

1. In an electron microscope comprising a threestage image formingelectron lens system consisting of an objective lens, an intermediatelens and a projection lens arranged in this order in the direction of anelectron beam axis, and excitation current sources each connected toeach of said electron lenses for supplying excitation currents theretoand for stabilizing said excitation currents, each of said excitationcurrent sources including means for adjusting said excitation currents;a focusing apparatus comprising means for mechanically coupling saidexcitation current adjusting means to simultaneously control saidexcitation cur rents, so that when the magnification of said electronmicroscope is varied by varying the excitation current of saidintermediate lens while keeping the excitation current, of saidprojection lens constant, the excitation current of said objective lensis'set at the value compensating forthe variation of said excitationcurrent of said intermediate lens to provide proper focusing.

2. In an electron microscope having a three-stage image producingelectron lens system consisting of an objective lens, an intermediatelens and a projective lens which are disposed in the above order in thedirection of the electron beam axis, an apparatus for attaining thefocusing following a variation in the magnification comprising anexcitation power supply connected to each of said electron lenses forsupplying excitation current to said electron lenses and stabilizing theexcitation current, each said excitation power supply including anexcitation current control means for controlling the excitation currentflowing through the lens coil, a detecting resistor for detecting theexcitation current flowing through said lens coil, means for varying theresistance of said resistor, a source of reference voltage for supplyinga reference voltage for comparison between'it and the voltage detectedby said detecting resistor, and an-amplifier for detecting andamplifying the difference between said detected voltage and the voltageof said reference voltage source, said amplifier having its first inputterminal connected to said detecting resistor, its second input terminalconnected to said reference voltage source and its output terminalconnected to said excitation current control means, and meansmechanically coupling said resistance varying means for said detectingresistors in said excitation power supplies for the simultaneous controlof the resistances of said detecting resistors so as to set theexcitation current for said objective lens at a value corresponding to avariation of the value of excitation current for said intermediate lenswhen the value .of excitation current for said intermediate lens isvaried while maintaining the excitation current for said projective lensat a constant value.

3. An apparatus for effecting focusing dependent upon the variation inmagnification of an electron microscope, comprising: an electron sourcefor producing electrons; an irradiation electron lens system including aplurality of condenser lenses for converging said electrons into anelectron beam to direct said electron beam to a specimen; a three-stageimage forming electron lens system consisting of an objective lens, anintermediate lens and a projection lens; excitation current sources eachconnected to each of said lenses for supplying excitation currentsthereto and for stabilizing said excitation currents, each of saidexcitation current sources including means for adjusting each respectiveexcitation current; and means for mechanically coupling said excitationcurrent adjusting means to simultaneously control said excitationcurrents, so that when the magnification of said electron microscope isvaried by varying the excitation current of said intermediate lens whilekeeping the excitation current of said projection lens constant, theexcitation current of said objective lens is set at the valuecompensating for the variation of said excitation current of saidintermediate lens to provide proper focusing necessitated by thevariation in the magnification and at the same time the excitationcurrents of said condenser lenses are set at the values corresponding tothe variation of said magnification to adjust the brightness dependentupon said variation in magnification.

4. In an electron microscope comprising a three stage image formingelectron lens system including an objective lens, an intermediate lens,and a projection lens arranged sequentially in the direction of theelectron beam axis, and excitation sources connected to each respectiveelectron lens for supplying excitation currents thereto and forstabilizing said excitation currents, each of said excitation sourcesincluding means for adjusting said excitation currents, an improvedfocusing apparatus comprising:

control means, coupled to each of said current sources and said lenses,for simultaneously maintaining the excitation current of said projectionlens constant,

for varying the excitation current of said intermediate lens, and

for adjusting the excitation current of said objective lens to apredetermined value to provide proper focusing in response to saidvariation of the excitation current of said intermediate lens, whereineach of said excitation sources comprises a power supply, a switchingtransistor connected between sar power supply and a respective electrondeflection element associated with each lens and a control circuitresponsive to said control means for appropriately energizing saidswitching transistor to control the flow of current to said deflectionelement, and wherein said deflection element comprises a lens coil andwherein said control circuit comprises a voltage dividing circuit,connected to said lens coil, and a differential amplifier having oneinput connected to a respective source of reference potential andanother input adjustably connected to said voltage dividing circuit, theoutput of said differential amplifier being connected to control theoperation of said switching transistor.

5. An improved focusing apparatus according to claim 4, wherein saidcontrol means further includes a mechanically ganged switchingarrangement having respective switches connected between each voltagedivider and the corresponding input to each of said differentialamplifiers.

6. A method of varying the magnification of an electron mic roscopehaving a four-stage image forming electron lens system consisting of anobjective lens, intermediate lens, and first and second projectionlenses arranged in this order in the direction of an electron beam axis,excitation current sources each connected to each of said electronlenses for supplying an excita- 1 tion current thereto and forstabilizing said excitation current, each of said excitation currentsources including means for adjusting said excitation current, and meansfor mechanically'coupling said excitation current adjusting means tosimultaneously control said excitation currents in a predeterminedrelation therebetween, comprising the steps of:

a. dividing the entire magnification range into a plurality of sectionsand varying the excitation current of said intermediate lensdiscontinuously at the boundaries of said sections while keeping saidexcitation current of said intermediate lens constant within eachsection;

. varying the excitation current of one of the electron lenses disposeddownstream of said intermediate lens within said each section to varythe magnification of said electron microscope; and

c. keeping the excitation current of said objective lens, within saideach section, substantially constant at avalue compensating for thevariation of the excitation current of said intermediate lens.

7. A method of varying the magnification of an electron microscopeaccording to claim 6, in which said one electron lens of the step (b) issaid first projection lens.

8. A method of varying the magnification of an electron microscopeaccording to claim 6, in which said one electron lens of the step (b) issaid second projection lens. 1

1. In an electron microscope comprising a three-stage image formingelectron lens system consisting of an objective lens, an intermediatelens and a projection lens arranged in this order in the direction of anelectron beam axis, and excitation current sources each connected toeach of said electron lenses for supplying excitation currents theretoand for stabilizing said excitation currents, each of said excitationcurrent sources including means for adjusting said excitation currents;a focusing apparatus comprising means for mechanically coupling saidexcitation current adjusting means to simultaneously control saidexcitation currents, so that when the magnification of said electronmicroscope is varied by varying the excitation current of saidintermediate lens while keeping the excitation current, of saidprojection lens constant, the excitation current of said objective lensis set at the value compensating for the variation of said excitationcurrent of said intermediate lens to provide proper focusing.
 1. In anelectron microscope comprising a three-stage image forming electron lenssystem consisting of an objective lens, an intermediate lens and aprojection lens arranged in this order in the direction of an electronbeam axis, and excitation current sources each connected to each of saidelectron lenses for supplying excitation currents thereto and forstabilizing said excitation currents, each of said excitation currentsources including means for adjusting said excitation currents; afocusing apparatus comprising means for mechanically coupling saidexcitation current adjusting means to simultaneously control saidexcitation currents, so that when the magnification of said electronmicroscope is varied by varying the excitation current of saidintermediate lens while keeping the excitation current, of saidprojection lens constant, the excitation current of said objective lensis set at the value compensating for the variation of said excitationcurrent of said intermediate lens to provide proper focusing.
 2. In anelectron microscope having a three-stage image producing electron lenssystem consisting of an objective lens, an intermediate lens and aprojective lens which are disposed in the above order in the directionof the electron beam axis, an apparatus for attaining the focusingfollowing a variation in the magnification comprising an excitationpower supply connected to each of said electron lenses for supplyingexcitation current to said electron lenses and stabilizing theexcitation current, each said excitation power supply including anexcitation current control means for controlling the excitation currentflowing through the lens coil, a detecting resistor for detecting theexcitation current flowing through said lens coil, means for varying theresistance of said resistor, a source of reference voltage for supplyinga reference voltage for comparison between it and the voltage detectedby said detecting resistor, and an amplifier for detecting andamplifying the difference between said detected voltage and the voltageof said reference voltage source, said amplifier having its first inputterminal connected to said detecting resistor, its second input terminalconnected to said reference voltage source and its output terminalconnected to said excitation current control means, and meansmechanically coupling said resistance varying means for said detectingresistors in said excitation power supplies for the simultaneous controlof the resistances of said detecting resistors so as to set theexcitation current for said objective lens at a value corresponding to avariation of the value of excitation current for said intermediate lenswhen the value of excitation current for said intermediate lens isvaried while maintaining the excitation current for said projective lensat a constant value.
 3. An apparatus for effecting focusing dependentupon the variation in magnification of an electron microscope,comprising: an electron source for producing electrons; an irradiationelectron lens system including a plurality of condenser lenses forconverging said electrons into an electron beam to direct said electronbeam to a specimen; a three-stage image forming electron lens systemconsisting of an objective lens, an intermediate lens and a projectionlens; excitation current sources each connected to each of said lensesfor supplying excitation currents thereto and for stabilizing saidexcitation currents, each of said excitation current sources includingmeans for adjusting each respective excitation current; and means formechanically coupling said excitation current adjusting means tosimultaneously control said excitation currents, so that when themagnification of said electron microscope is varied by varying theexcitation current of said intermediate lens while keeping theexcitation current of said projection lens constant, the excitationcurrent of said objective lens is set at the value compensating for thevariation of said excitation current of said intermediate lens toprovide proper focusing necessitated by the variation in themagnification and at the same time the excitation currents of saidcondenser lenses are set at the values corresponding to the variation ofsaid magnification to adjust the brightness dependent upon saidvariation in magnification.
 4. In an electron microscope comprising athree stage image forming electron lens system including an objectivelens, an intermediate lens, and a projection lens arranged sequentiallyin the direction of the electron beam axis, and excitation sourcesconnected to each respective electron lens for supplying excitationcurrents thereto and for stabilizing said excitation currents, each ofsaid excitation sources including means for adjusting said excitationcurrents, an improved focusing apparatus comprising: control means,coupled to each of said current sources and said lenses, forsimultaneously maintaining the excitation current of said projectionlens constant, for varying the excitation current of said intermediatelens, and for adjusting the excitation current of said objective lens toa predetermined value to provide proper focusing in response to saidvariation of the excitation current of said intermediate lens, whereineach of said excitation sources comprises a power supply, a switchingtransistor connected between said power supply and a respective electrondeflection element associated with each lens and a control circuitresponsive to said control means for appropriately energizing saidswitching transistor to control the flow of current to said deflectionelement, and wherein said deflection element comprises a lens coil andwherein said control circuit comprises a voltage dividing circuit,connected to said lens coil, and a differential amplifier having oneinput connected to a respective source of reference potential andanother input adjustably connected to said voltage dividing circuit, theoutput of said differential amplifier being connected to control theoperation of said switching transistor.
 5. An improved focusingapparatus according to claim 4, wherein said control means furtherincludes a mechanically ganged switching arrangement having respectiveswitches connected between each voltage divider and the correspondinginput to each of said differential amplifiers.
 6. A method of varyingthe magnification of an electron microscope having a four-stage imageforming electron lens system consisting of an objective lens,intermediate lens, and first and second projection lenses arranged inthis order in the direction of an electron beam axis, excitation currentsources each connected to each of said electron lenses for supplying anexcitation current thereto and for stabilizing said excitation current,each of said excitation current sources including means for adjustingsaid excitation current, and means for mechanically coupling saidexcitation current adjusting means to simultaneously control saidexcitation currents in a predetermined relation therebetween, comprisingthe steps of: a. dividing the entire magnification range into aplurality of sections and varying the excitation current of saidintermediate lens discontinuously at the boundaries of said sectionswhile keeping said excitation current of said intermediate lens constantwithin each section; b. varying the excitation current of one of theelectron lenses disposed downstream of said intermediate lens withinsaid each section to vary the magnification of said electron microscope;and c. keeping the excitation current of said objective lens, withinsaid each section, substantially constant at a value compensating forthe variation of the excitation current of said intermediate lens.
 7. Amethod of varying the magnification of an electron microscope accordingto claim 6, in which said one electron lens of the step (b) is saidfirst projection lens.